package/zlib: allow shared-only builds

Currently, we can build a static-only zlib, but not a shared-only one.

This is because zlib's build-system is a custom ./configure (not using
autotools), and does not allow building/installing only the shared
library.

Simply remove the .a as a post-staging install hook. We don't care
removing it from target, since it is not used at link time to build
other packages, and it is anyway removed later before assembling the
filesystem images anyway.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>